1. Field of the Invention
The present invention relates to a front teleconverter lens system for varying the focal length of a whole lens system to the longer side by attaching to the object side of an imaging lens.
2. Description of Related Art
A front teleconverter that is attached to the object side of an imaging lens for varying the focal length of the whole lens system to the longer side has been known to be used for a still camera, a video camera, and the like. Generally, this kind of front teleconverter is composed of, in order from the object, a first lens group having positive refractive power, and a second lens group having negative refractive power, where the focal points of respective lens groups are arranged to substantially coincide with each other forming an afocal lens system that collimated light incident to the object side exits as collimated light. An example of the conventional front teleconverter lens system that is composed of, in order from the object, a single positive lens having positive refractive power and a single negative lens having negative refractive power is disclosed in Japanese Laid-Open Patent Application No. 63-210810. The construction of the lens is relatively simple.
Although the conventional front teleconverter lens system disclosed in the above-described document can be compact and lightweight with fewer lens and simple construction, aberration becomes slightly large, so that optical performance is not sufficient.